An investigation is underway into whether Sacramento police used excessive force against an unarmed black man accused of jaywalking. (Reuters)

When Nandi Cain Jr. was seen on video getting slammed to the ground and pummeled repeatedly by a police officer, that was not the end of his ordeal, according to his attorney.

A new federal lawsuit states that the 24-year-old was also placed on psychiatric hold and taken to an isolation cell of a county jail, where the officer and other employees beat him repeatedly, stripped his clothes off and made obscene comments. Cain was then left in the cell, where he spent hours without food, medical attention or a chance to make a phone call, the lawsuit says.

Cain filed a lawsuit Sunday alleging violation of his constitutional rights. His attorney, John Burris, said the officer arrested Cain simply because he’s black, and later “engaged in dehumanizing and derogatory conduct.”

“He shouldn’t have been in psychiatric watch,” Burris told The Washington Post. “He shouldn’t have been in the jail in the first place.”

Cain was arrested a few miles outside of downtown Sacramento in Northern California on April 10, when Officer Anthony Figueroa approached him while he was walking home from work. Cain put his hands up, but continued to walk and asked the police officer why he was being stopped.

Figueroa then stepped forward and grabbed Cain. The officer slammed him to the ground, straddled him and punched him repeatedly — about 20 blows to the head and face, according to the lawsuit. Soon other officers sped to the scene and helped subdue Cain.

The encounter was caught on video by several police dash cams and the cellphone of an acquaintance of Cain’s who was passing by. A video inside the patrol car shows a handcuffed Cain kicking the seats repeatedly.

The police department has since started a criminal investigation into the officer’s conduct. A separate internal affairs investigation is also pending.

Figueroa, a two-year member of the Sacramento police force, has been placed on administrative leave with pay. It was not immediately clear whether he has an attorney.

“The actions of the involved Sacramento police officer are disturbing and does not appear to be reasonable based upon the circumstances,” the agency said in a statement this month.

But according to Burris, Cain’s attorney, the abuse did not end on the street.

He was then asked after he arrived at the jail whether he was suicidal, to which he repeatedly replied, “No,” the lawsuit says.

Cain was nevertheless taken to an isolation cell. There, Figueroa allegedly told him to lie down with his face and body facing the concrete. Later, the officer called several other jail employees, and the men repeatedly kneed Cain’s ribs and forcibly took his clothes off, the lawsuit says.

The men called him a “b—-” and told him his naked body “stank,” the lawsuit claims.

Cain was later left alone in the cell, where he stayed for nine hours. He was released about 2 a.m. He was never charged.

The lawsuit was filed against Figueroa, the city and county of Sacramento and unnamed jail employees.

A spokesman for the police department said the agency cannot comment, citing the pending litigation.

Sgt. Tony Turnbull, spokesman for the Sacramento County Sheriff’s Department, said the agency will investigate the allegations of abuse at the jail.

“I would like to reiterate that these are only allegations,” Turnbull said. “We feel the truth will come out at the conclusion of the investigation in regards to Mr. Cain’s allegations against our department.”

Burris said he’s hoping to find out if there was a surveillance video in the isolation cell.

He said his client suffered a concussion, a black eye and several bumps and bruises. Cain has also had nightmares every night since the incident, Burris said.

“He’s been unable to sleep. He’s been crying off and on,” Burris said. “He’s not been able to understand and appreciate why the officer did this to him, other than the fact that he’s a young black guy walking down the street.”

The night Cain was arrested, 40 demonstrators gathered in front of one of the police department’s substations, many of whom wore “Black Lives Matter” shirts, the Sacramento Bee reported.

The case occurred as police department across the United States are under increased scrutiny amid claims that they are too quick to use force against black people and other minorities.

Earlier this month, U.S. Attorney General Jeff Sessions ordered the Justice Department to review reform agreements at police departments across the nation. The review includes some consent decrees that encouraged police departments to train their officers on how to better resolve situations without using force, according to The Washington Post’s Sari Horwitz, Mark Berman and Wesley Lowery.